An example: I flew to the 'states this weekend. Once on the ground, I topped up my American (T-Mobile) sim with a month's worth of credit. (There was no way to do this from abroad since their sims don't work there at all, and their website has a ridiculous country redirect even from your account page).
At the airport, on the way home, I sent a text to my wife letting her know I was on the way. It failed.
I tried again to various permutations of the French number, with absolutely no success. Googling around, it seems that you can't do that with t-mobile. You can't even get them to turn it on, since their "international" packages only work with their contract services. Not just roaming, but even placing a phone call to another country is impossible with pre-paid t-mobile. Sure enough, landing in Paris, the phone was dead dead dead. No "welcome to France" message. No extortionate international roaming charges. Just no ability to make calls at all.
I had to pop my UK sim back in just to be able to use the phone again.
I'm looking forward to the first company (in any country) that truly gets all this. They'll get my business, as well as pretty much all the business from anybody who travels at all.